Rates of reaction

4.17   describe experiments to investigate the effects of changes in surface area of a solid, concentration of solutions, temperature and the use of a catalyst on the rate of a reaction

Examples of such methods and experiments you should know are:

i     )Measuring the volume of gas given off

ii)   Measuring a mass loss

iii)   The decomposition of hydrogen peroxide using a catalyst

iv)  Interpreting graphs showing changes of mass and volume against time

v)   Rate graphs, showing change in rate with concentration or temperature

vi)  The reaction between sodium thiosulfate and hydrochloric acid

vii)  Keeping things fair

Study the examples on the following pages and look back to your own work on this topic.i      )Measuring the volume of gas given off

An example reaction is that between zinc and dilute hydrochloric acid:

Zn(s) + 2HCl(aq) → ZnCl2(aq) + H2(g)

We could change the mass of zinc used, the size of zinc granules (surface area), the concentration of the hydrochloric acid, the volume of the hydrochloric acid or the temperature of the hydrochloric acid.  All these things are independent variables.  Whichever one we want to study, we need to keep all the others exactly the same, as they could all have an effect on the rate of reaction.  This is what is meant by “a fair test” (see part vii) later).

The volume of gas is often measured with a gas syringe.  Alternatively you could collect the hydrogen gas over water in an inverted measuring cylinder.

ii)    Measuring a mass loss

An example reaction is that between marble chips and dilute hydrochloric acid:

CaCO3(s) + 2HCl(aq) → CaCl2(aq) + CO2(g) + H2O(l)

One could change the mass of marble used, the size of marble chips (surface area), the concentration of the hydrochloric acid, the volume of hydrochloric acid or the temperature of the hydrochloric acid.

If a gas is given off, the mass of the system will decrease with time, so a reaction can be carried out in a conical flask placed on a balance, plotting mass against time.

A loose plug of cotton wool is put in the neck of the conical flask.  This allows the carbon dioxide gas to escape, which we want, but stops a spray of acid leaving the flask, which would give erroneous results (the recorded mass loss would be too great).

iii)   The decomposition of hydrogen peroxide  (See also Section 2:18)

Hydrogen peroxide decomposes very slowly to give water and oxygen:

2H2O2(aq) → 2H2O(l) + O2(g)

The reaction can be speeded up by using a catalyst, such as the black solid, manganese(IV) oxide, MnO2.  Remember that the oxygen produced comes from the hydrogen peroxide, not the catalyst.  The catalyst is not used up or changed chemically during the reaction.

It is usual to measure the volume of oxygen produced with time, using a gas syringe.

It is possible to study the effects of varying the temperature of the hydrogen peroxide, or the concentration or volume of it, but the mass and surface area of the catalyst must stay the same.  We can also study the effectiveness of different catalysts and different amounts of catalyst.  In this case the temperature, volume and concentration of the hydrogen peroxide must stay the same.  We could, for example, measure how long it takes to produce a certain volume of gas in each case.

iv)   Interpreting graphs showing changes of mass or volume

You need to be able to interpret the graphs met so far, as in the following example.

The graph shows mass against time for the reaction between marble chips and hydrochloric acid at two concentrations, 1 mol/dm3 and 2 mol/dm3.  Everything else was kept the same.

The marble chips were in excess (so the acid runs out at the end of the experiment, not the marble).

The curve for the 2 mol/dm3 acid is steeper and becomes horizontal sooner because the rate is faster at the higher concentration, so the reaction stops sooner.

The curve for the 2 mol/dm3 and has twice the total mass loss of the 1 mol/dm3 experiment. This is because at double the concentration, there is double the amount of acid to react, so twice the volume of gas is produced.

If we had changed temperature, or marble chip size, the graphs would have finished at the same height, as the amounts of reactants would have been the same.  There would be differences in the gradients of the graphs at the start, however.

v)         Rate graphs

The graphs seen so far have all been plots of masses or volumes followed with time.

Another technique is to measure the time for something to occur in one experiment (such as the time taken for a piece of Mg ribbon to disappear), and then to repeat the experiment several times with different concentrations or temperatures.

The times are easily converted to rates, (rate = 1 ÷ time), and can be plotted as shown below.

You need to be able to describe the relationships shown by graphs.  A basic description would be to say “the rate increases with concentration” or “the rate increases with temperature”.  Sometimes you need to be more precise and describe the mathematical relationship between the two, e.g., in the graphs above:

Rate is directly proportional to concentration.  Rate and temperature have an exponential relationship.

vi)   The reaction between sodium thiosulfate and hydrochloric acid

This is a suitable experiment to get data to plot rate graphs, as shown above.

When these two solutions react, a yellow precipitate of sulphur forms, and it is no longer possible to see through the mixture.  This can be carried out in a conical flask or beaker on top of a piece of paper marked with a cross.  The time it takes for the cross to be obscured by the sulphur precipitate is a good way of comparing rates.

This can be repeated with different concentrations of acid (or temperatures) giving a series of times, which can be converted to rates as below:

concentration of acid / mol/dm3	0.25	0.50	0.75	1.00	1.25
time for cross to be obscured / s	23.8	12.4	8.2	6.0	4.9
rate of reaction / s-1	0.0421	0.0802	0.122	0.167	0.204

vii) Keeping things fair

Whichever rate experiment we do, we only ever change one thing at a time, so if we are changing concentration, for example, we would keep the same volumes of liquids, mass of solids, size of solid particles and temperature.